The invention relates generally to sensors for detecting substances, and in particular to sensors for detecting gases in the presence of humidity.
Chemical sensing of gases is an important technology in several fields including environmental monitoring, industrial safety, and public security. Depending on the application and target gas, different operating principles have been deployed, ranging from electrochemical, metal-oxide semiconductor, and non-dispersive infrared absorption. With the recent advancement of smartphones, wearables, and connected sensor devices, many more chemical sensing applications have emerged. In particular monitoring the health, safety and well being of consumers is receiving considerable interest. The problem is that existing technologies listed above do not meet the needs of these emerging applications. Specifically, existing devices lack specificity and have a relatively large size, power requirement and cost.
One problem to overcome is that resonant chemical sensors can be highly sensitive to even moderate changes in relative humidity. When measuring an analyte in ambient environmental conditions, the sensor indicates not only a mass change due to the target gas, but also due to the additional adsorption of water molecules in the sensing material. While many sensing materials can be tailored to detect specific analyte molecules with low cross sensitivity, the absorption of water molecules is a more difficult problem to solve due its omnipresence and chemical activity.
There have been multiple approaches documented in the literature to address the problem of water interference for chemical sensing. These approaches include the design of a hydrophobic receptor material, the incorporation of filters and membranes to block water access, the addition of a second sensor with different water affinities (as described in US 20130311108 A1), or adding a separate humidity sensor as a reference (as described in U.S. Pat. No. 7,887,683 B2). Each of these methods has serious limitations. For example, a hydrophobic receptor material—if it can be found—often has reduced sensitivity for the analyte due to the trade off in the material characteristics. The addition of a filter or membrane increases cost and raises the issue of saturation, desorption and hysteresis of the filter which can affect the measurement and longevity of the device. Adding a separate relative humidity sensor fails because commercially available humidity sensors lack the required precision and accuracy. Adding a second sensing material to monitor water adsorption is problematic because the two sensing materials are not exactly identical and the differences between them causes errors too large to overcome, especially over time. Finally, most of these approaches increase cost, size and complexity. There is still a need for a simple, low cost, miniaturized sensor to detect analyte that accounts for the effects of humidity and water adsorption by the sensing material.